Device for imaging radiation

ABSTRACT

An imaging device for radiation imaging is an array of image cells. The array of image cells consists of an array of detector cells and an array of image cell circuits. Each detector cell is connected to the corresponding cell in the array of image cell circuits. Each individual cell in the detector cell array generates a charge based on the radiation that hits the cell. Each cell in the array of image cell circuits accumulates the charge on a storage capacitor. The storage capacitor can be, for example, the gate of a transistor. Single cells in the array of image cells can be grouped together to form larger area super cells. The size of the super cell can be controlled by control signals, which select the operating mode. The output from a cell, a single cell or a super cell, is read out in current mode and is scaled according to the size of the cell. Several modes can be implemented in the imaging device. Also, an imaging system for larger area radiation imaging can be implemented by connecting several imaging devices together in form of a two-dimensional array. The array of cell circuits comprises separate enable signals for selecting individual rows and columns and output signals indicating an end of row and an end of column.

FIELD OF INVENTION

This invention relates to radiation imaging using a semiconductorimaging device consisting of an array of image cells.

BACKGROUND TO INVENTION

This invention describes a semiconductor imaging device for radiationimaging. The imaging device is an array of image cells, which consistsof an array of radiation detector cells and an array of image cellcircuits. An example of an imaging system configuration is shown in FIG.1 of the accompanying drawings. All cells in the detector cell array areconnected to respective electronics cells in the array of image cellcircuits. With appropriate processing technology, it is possible toimplement both detector cells and circuit cells on the same substrate.Another possibility is to have two substrates, one for the detector andone for the cell circuits and, by using a bump-bonding or othertechnique connect them mechanically and electrically together so thateach detector cell is connected to the corresponding cell circuit. Across-section of a part of an imaging device made of two substrates,which are bump-bonded together, is shown in FIG. 2 of the accompanyingdrawings.

In many radiation imaging applications, a need for different imageresolutions exist. In single exposure images, the resolution shouldusually be relatively high. On the other hand, the same imaging systemcould be used for displaying live image by continuously reading theimage from the imaging device and updating the display in real time.However, if the imaging system is designed for high resolution, the databandwidth for a live image at, for example, 30 frames per second may beso high that the requirements for the readout electronics for handlingthe data stream may become unreasonable. A readout system fast enough tocapture and process the images could become unreasonably expensivecompared to the total cost of the imaging system. Furthermore, a highimage resolution required for single exposure images may not even berequired for a live display of images.

Therefore, a method for effectively reducing the resolution and thus thedata bandwidth on chip would solve the problem. Another problem is thescalability of the imaging system for large or small area imagingsystems. If single imaging devices with relatively small area could beeasily linked together to form a seamlessly connected array of imagingdevices for large area imaging system, the same imaging devices couldeasily be used for either large and small area applications.

This invention tries to solve the problems addressed above byintroducing an imaging device with programmable image resolution andsimple tiling of the devices to make a flexible imaging system for widevariety of target applications.

Particular and preferred aspects of the invention are set out in theaccompanying independent and dependent claims. Features from dependentclaims may be combined with those of the independent claims in anyappropriate manner and not merely in the specific combinationsenumerated in the claims.

SUMMARY OF INVENTION

In accordance with one aspect of the invention there is provided, animaging device for radiation imaging, said device comprising an array ofdetector cells for generating a charge in response to incidentradiation, an array of cell circuits for accumulating charge generated,and control circuitry controlling output of signals from said cellcircuits, said control circuit operable to select an individual columnand row of said array of cell circuits.

The array of detector cells and array of cell circuits form an array ofpixels. As a result of the control circuits being operable tocontrollably select separate columns and rows, the resolution of thepixel array can be programmed.

In a preferred embodiment, the control circuitry is further operable tooutput separate enable signals for selecting said individual column androw. Thus providing individual selectability of columns and rows.

In a preferred embodiment, the said control circuitry is furtheroperable to output a signal indicative of an end of row and a signalindicative of an end of column for said imaging device, therebyfacilitating coupling separate devices together to form a large areadetector.

In a preferred embodiment, the end of row and end of column outputsignals of a first device are connected to corresponding enable signalsof an adjacent device in first and second orthogonal directions,respectively, to form an array of imaging devices for larger arearadiation imaging.

In another aspect of the invention there is provided an imaging system,comprising a plurality of imaging devices according to any precedingclaim connected as a one or two-dimensional array.

In a preferred embodiment the imaging system comprises a plurality ofimaging devices according to any preceding Claim connected as atwo-dimensional array, whereby said imaging system provides selectableimaging resolutions for selected applications.

Suitably, the control circuitry is arranged to permit reading of cellcircuits of one row of pixels across multiple imaging devices from thetwo-dimensional array of imaging devices, before proceeding to asubsequent row.

In an embodiment of the invention, there is provided an imaging devicefor radiation imaging, the device comprising an array of detector cellsfor generating a charge in response to incident radiation, an array ofcell circuits for accumulating charge generated, and control circuitrycontrolling output of signals from the cell circuits programmable toadjust the resolution of the imaging device.

As a result of the programmable resolution, an imaging device accordingto an embodiment of the invention can provide different operationalmodes giving different pixel resolutions for different targetapplications.

In a preferred embodiment, the programmability in that the controlcircuitry is arranged to select a group of cell circuits and to producean output signal representative of a sum of charge accumulated in allcell circuits in a group. Thus, the control circuitry enables groupingof several pixels together to form a larger area super pixel for lowerresolution imaging.

In a preferred embodiment the control circuitry averages signalsrepresentative of charge accumulated in all cell circuits in a group.For example, the output signal is representative of the total charge forall of the cell circuits of a group divided by the number of cellcircuits in the group. Preferably, the number of cell circuits in agroup is selectable from a set of possible numbers.

In a preferred embodiment, the output signal representative of chargeaccumulated is a current value. The use of a current output facilitatescircuitry required to combine and average signal levels.

The control circuitry, for selecting a group of cell circuits, comprisesa shift register arranged to select a plurality of columns or rowsconcurrently and to advance in steps of more than one row or column. Thecontrol circuitry can additionally comprise logic arranged simultaneousto select a plurality of rows and columns and a step size larger thanone.

The control circuitry is arranged to average currents from a group ofcell circuits by connecting current outputs of each cell circuit into acommon output node and dividing the resulting sum of currents by thenumber of pixels in the group using a current mirror. The common outputnode can hold a current of the selected cell circuit(s).

Alternatively, for implementing group modes, each cell circuits in agroup can be arranged to produce a scaled output signal representativeof charge accumulated in the cell circuit divided by a number of cellcircuits in the group. In order to be operable in a plurality of groupmodes, where each group mode has associated with it a predeterminednumber of cell circuits, the cell circuits can be arranged to include anoutput transistor for each group mode, which output transistor producesa scaled output signal according to the number of cells in a selectedgroup mode. The output signal from all cell circuits in the group canthen be averaged by summing the signals together.

In an embodiment of the invention, the resolution is controlled fromoutside by one or more control signals. For example, with two controlsignals, four different modes for resolution can be achieved. Thus,separate enabling signals can be provided for selecting columns and rowsand output signals for indicating end of row or end of column.

Thus, in an embodiment of the invention, in addition to a mode whereevery individual pixel is read, 2×2, 3×3 or 4×4 pixels could be groupedtogether and read out as super pixels. Other pixel combinations (forexample having different numbers of rows and columns) and differentnumber of modes can be used as well. The summation of pixel values canbe easily done since the summation is done in current mode. Outputcurrents of several cells are connected together. Adding currents fromseveral cells together results in larger overall current. This can becompensated by an additional current mirror, which scales the currentoutput to the same range as the current output of a single cell. Inother words, the current mirror divides the current from a super pixelby the number of cells in the super pixel. This is equivalent to takingan average of a larger number of individual pixels. Using current modeoutput also has another advantage, enabling longer wiring without losingaccuracy. Performing the averaging of pixel values is by no meanslimited to using current output. Voltage mode can be used instead of thecurrent mode as described hereinafter. Using voltage mode would requirethe voltages of several pixels to be summed and averaged by using, forexample, an op-amp circuit.

Moreover, an embodiment of the invention thus provides a solution to theproblem of providing a video scan output from a imaging deviceconstructed from a plurality of readout devices. Thus rather thanreading out a device at a time, a large area imaging system formed fromsmall area readout devices can be read one line from the whole imagingarea before advancing to the next row of pixels. Together the readoutdevices form a seamless large area imaging system enabling scannedoutput over the whole image area. The imaging device has two inputsignals, which start the sequence for selecting the column and row foroutput. Furthermore, the imaging device has two outputs, one of whichindicate when the last pixel of each line has been read and the otherindicates when the last row of the device has been read. These outputsignals are connected to the corresponding input signals in the adjacentimaging devices in horizontal and vertical direction. The column and rowoutput from the last imaging device can be connected to the first deviceto make the system run in a continuous mode for live video applications.The mode is selectable so that the user can switch between the singleexposure mode and the live video mode at any time.

The combination of the above mentioned features makes it possible to usethe same system for making single exposures with high resolution and atany time switching to live video mode and at the same time changing tolower resolution to reduce the data bandwidth. The size of the pixels isnot fixed to any physical dimensions, but can be scaled according toavailable processing technology and based on the requirements of thetarget application.

The invention also provides an imaging system, comprising a plurality ofimaging devices according to as defined above connected as atwo-dimensional array, whereby the imaging system provides selectableimaging resolutions for selected applications. Control circuitry canpermit reading of cell circuits one row at a time from thetwo-dimensional array of imaging devices, as opposed to one imagingdevice at a time.

In accordance with another embodiment of the invention, there isprovided a method of operating an imaging device for radiation imaging,which device comprises of an array of detector cells for generating acharge in response to incident radiation, an array of cell circuits foraccumulating charge generated, and control circuitry controlling outputof signals from the cell circuits, the method comprising:

selecting a resolution of the imaging device;

adjusting addressing of the cell circuits to group outputs from the cellcircuits according to a selected resolution.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an overall imaging system for radiation imaging.

FIG. 2 depicts an example of a cross-section of an imaging devicecomprising of a pixel array.

FIG. 3 shows a schematic diagram of one cell of image electronics.

FIG. 4 illustrates the grouping of pixels into larger area super pixels.

FIG. 5 shows a two-dimensional pixel array with control signals forselecting output pixels.

FIG. 6 shows an example of a schematic diagram used for selectingcolumns in a pixel array.

FIG. 7 shows an example of a schematic diagram used for selecting rowsand resetting rows of pixels in a pixel array.

FIG. 8 illustrates imaging system consisting of an array of imagingdevices and with two modes of operation: single shot and continuous.

DETAILED DESCRIPTION OF THE EMBODIMENTS OF INVENTION

Exemplary embodiments are described by way of example only withreference to the accompanying drawings.

FIG. 1 shows an example of an imaging system for radiation imagingincluding an imaging device according to the invention. The imagingdevice is intended for imaging of high-energy radiation, for exampleX-ray radiation. However, the invention is not limited to imaging ofhigh-energy X-ray radiation, but can be applied to detection of any typeof radiation, for example a-ray, b-ray, g-ray, infra red or opticalradiation, subject to choice of appropriate semiconductor substrate forthe detector.

The imaging system 10 in FIG. 1 provides imaging of an object 12 subjectto radiation 14. The object may, for example, be part of a human body incase of medical imaging or any other object, in case of non-destructivetesting.

The imaging device 16 in FIG. 1 can consist of one or two semiconductorsubstrates. In case of one substrate, each cell 18 in the substratecomprises of a pixel detector and a pixel circuit. Alternatively, twosubstrates can be used, one containing an array of detector cells whilean array of pixel circuits is located on another substrate. The twosubstrates can, for example, be connected using a bump-bonding or othertechnique, as described below.

Each detector cell on the imaging device 16 detects high-energyradiation and generates a charge, which is accumulated on a capacitor inthe corresponding image cell circuit. After a certain iteration time,the charge is read out from the cell circuits as a currentrepresentative of stored charge, one cell at a time. The controlelectronics 20 create the necessary signals for starting the iterationand resetting the cells to a predefined value after the iterated chargehas been read. The current from each cell is amplified and scaled beforeconverting it to a digital signal, or word in the image acquisition unit22. The digital information is further processed in the image processingunit 22 to create a desired result. For example, calibration ofindividual pixels can be done in the image processing unit 22 in orderto compensate for the non-uniform response of pixels in the array.Process variations in fabrication of the detector array or theelectronics array may lead to pixels having a non-uniform response to auniform level of radiation. This can be compensated by post-processingof the image before displaying it on the display 24. The controlelectronics 20, image acquisition and processing unit 22 and the imagedisplay unit 24 can all be located inside a computer running applicationsoftware 26, which controls the whole system according to user inputsvia input devices 28, such as keyboard or mouse.

FIG. 2 is a cross-section of part of an imaging device. The imagingdevice consists of a detector substrate 30 and a readout substrate 32.In FIG. 2, the two substrates are connected together by a bump-bondingtechnique. The detector and readout substrates consist of an array ofdetector cells 34 and an array of cell circuits 36, respectively. Thedetector cell and the cell circuit form an image cell 38. The detectionarea of a detector cell 34 for the imaging cell 38 is defined between acontinuous electrode 40 and by a pixel electrode 42. The continuouselectrode on the detector substrate is used for applying a bias voltage.On the image electronics substrate, the contacts 44 for the pixel cellsare at the corresponding locations to the electrodes on the detectorsubstrate. A detector cell and the corresponding cell circuit areconnected by means of a bump-bond 46.

The physical size of the image cell 38, consisting of a pixel detectorcell 34 and corresponding circuit cell 36, is not fixed but can bedesigned according to the requirements of the target application andwithin the limits of available processing technology for integratedcircuit manufacturing. Also, with an appropriate semiconductor process,detector cells 34 and the corresponding cell circuits 36 can beimplemented on a same substrate. Thus, with suitable technology, theinvention is applicable to single substrate implementation as well asthe dual substrate technique described herein.

The material for the detector substrate 30 and readout substrate 32 canbe chosen according to the application and availability of suitableprocessing technologies. For example, silicon can be used for bothsubstrates. Other materials can be used as well. For example, thedetector substrate could be fabricated of CdZnTe, CdTe, HgI2, InSb,GaAs, Ge, TlBr, Si and PbI.

FIG. 3 depicts a schematic diagram of the image cell circuit 50. Eachpixel or image cell in the array comprises a similar cell circuit. InFIG. 3, the detector cell is represented by 52. The input of the cellcircuit, node 54 corresponds to the bump-bonded connection between thedetector cell and the cell circuit. When radiation ionizes the detectionzone in the detector, an electronic charge is created and accumulated onthe gate capacitance of the memory transistor 56. Two transistors 58 areused as switches between the drain of the memory transistor 56 and theoutput node of the cell 60. When the column 62 and row indicator signals64 for the cell are active concurrently, the drain of the transistor 56is connected to the output node 60 of the cell and the drain current 66of the memory transistor can be read out. The drain current is afunction of the gate-source voltage of the transistor and thusrepresents the charge accumulated on the gate capacitance of thetransistor 56.

Overflow of the gate voltage is protected by a diode 68 connectedbetween the gate of the memory transistor 56 and V₁ 70. Similarly,underflow is protected with a diode 72 between ground (GND) 74 and thegate of the transistor 56. An additional transistor 76 is used forresetting the gate voltage of the memory transistor 56 to a predefinedreset voltage value V_(reset) 78 every time the reset signal 79 isactive.

The grouping of pixels into larger area super pixels is represented inFIG. 4. Three imaging devices 80, 82 and 84 are shown in the FIG. 4. Theimaging device 80 illustrates a device with no pixel grouping. The imageis read one pixel 86 at a time. Using control signals for selecting themode of operation, the pixels in imaging devices can be grouped in tolarger clusters. FIG. 4 illustrates three different modes of operation,imaging device 80 with no grouping, imaging device 82 with grouping of2(2 pixels into one super pixel 88 and imaging device 84 with groupingof 4(4 pixels into one super pixel 90. The number of modes used in anactual application is by no means limited to the ones shown in thisexample but can be freely chosen according to the requirements of aparticular application.

With further reference to FIG. 3, if the imaging device is used in agroup mode, the output currents from all the cells in the group can besummed together and divided by the number of cells in the group in orderto produce an averaged output. An alternative way of implementing thesame averaging is to make the division on the cell itself before summingthe current outputs together. In order to do this, several memorytransistors 56 having different characteristics can be implemented ineach cell circuit. Thus, each cell can produce several outputs withdifferent scaling values dependent on different group sizes. In eachgroup mode, a different output is selected by a selection signal (notshown) according to the size of the group. The output currents from thecell circuits, which are already divided by the number of pixels in thegroup, is then summed to produce an averaged output.

FIG. 5 illustrates two identical imaging devices consisting of atwo-dimensional pixel array. Also, control signals for selecting theoutput pixel(s) are shown in FIG. 5. The array is of size M×N, where Mis the number of pixels in vertical direction and N is the number ofpixels in horizontal direction. Imaging device 100 in FIG. 5a usesoperating mode in which every pixel is read, i.e. no grouping of pixelsis used. The imaging device 102 in FIG. 5b is operating in a mode, whichgroups 2×2 pixels into one super pixel. Pixels 104 in FIG. 5 areindicated as Y,X, where X and Y represent the horizontal and verticallocation of the pixel in the two-dimensional pixel array, respectively.The imaging device has control logic which generates the requiredsignals for selecting the right column and row in the pixel array,according to the selected mode. The mode is selected with the modesignal 106. In FIG. 5a, the control logic 108 generates the signal,which selects one row at a time, starting from the first row. While thefirst row is selected, the control logic 110 generates a signal, whichselects the first column. On each clock cycle, the column selector 110advances to the next column until last column of a row is reached. Whenthe last column of the first row is finished reading, the row selector108 is advanced to the next row. This is repeated until last row isread. The resetting of the pixels can be done one row at a time, so thatthe reset signal generated for the row is the row-selecting signaldelayed by one clock cycle.

The imaging device 102 in FIG. 5b operates in a different mode comparedto the device 100 in FIG. 5b. In this mode two columns and two rows areselected simultaneously. The operation of the row selector 112 isidentical to the operation of row selector 108 in FIG. 5a except for theoperating mode, where two rows are selected simultaneously and theselector is advanced in two row steps. Similarly, the column selector114 operates in the same way as the column selector 110 except forselecting two columns at a time and advancing in two column steps. Thecontrol logic for row and column selectors can be designed to include asmany operating modes for grouping pixels as necessary for the targetapplication. In this example only two modes with no grouping andgrouping of 2×2 pixels were shown, but the number of built-in modes isnot limited in any way. Inside the control logic boxes 108, 110, 112 and114 in FIG. 5, the control signals are shown at various instants intime, indicating the selection of pixels. When both the column and rowselector for a pixel is selected, its output current can be read in theoutput node of the device.

FIG. 6 illustrates an example schematic diagram for the control logic120 that is used for selecting columns of a pixel array. A pixel isselected by selecting a row and a column simultaneously. The controllogic 120 in FIG. 6 consists of building blocks 122, each of whichcontain the necessary logic for selecting one or two columns in a twocolumn group. The building blocks form a shift register with therequired logic to enable grouping of pixels. Counters or other logic,which perform the same function can be used for selecting rows orcolumns as well. Two modes of operation are included in the logic inFIG. 6. The two modes are: no grouping and groups of two columns. Themode is controlled by a control signal 124. When the mode signal 124 isin logic low level, the logic operates in normal mode, selecting onecolumn 126 at a time and advancing to the next column at each clockcycle 128. The sequence is started by the col_ena signal 130. At thefirst clock cycle the state of the col_ena signal 130 is stored in theflip-flop 132, selecting first column. In this mode, the signalpropagates through the gate 133 and the output of the first flip-flop isconnected to the input of the second flip-flop. In each building blockin the control logic 120, the signals are connected identically. Onecolumn is selected at a time and at each clock cycle the signalpropagates to the next flip-flop selecting the next column in the chain.When the last column is selected, the control logic 120 produces anoutput signal col_out 134 so that one device can be connected to anotherto form a continuous array as explained later. The inverter 135 is addedto invert the clock signal 128 in order to produce the col-out outputsignal at the correct time so that the sequence of selecting columns iscontinued in the next imaging device without delay.

If the mode signal 124 is in logic high state, the control logic 120operates in a grouping mode where two columns are selectedsimultaneously and advanced two columns at a time at each clock cycle128. In this mode the signal propagates through the gate 136 instead ofgate 133, so that the input of the first flip-flop 132 is also the inputfor the next flip-flop. This enables the flip-flops to change statesimultaneously, selecting two columns at a time. In this mode theoperation of the control logic 120 is identical to the operation of thecontrol logic 138, where the selection signals of two successive rowsare connected together. When similar logic is used for selecting rows,this mode selects groups of 2×2 pixels. In this example only two modesof operation are available, but the number of modes is by no meanslimited to these modes. Any number and combination of different modescan be included in the row and column selecting logic using the sameprinciple as shown in FIG. 6. The number of mode inputs depends on thenumber of modes included in the design.

FIG. 7 illustrates an example schematic diagram for the control logic140 that is used for selecting rows from an array of pixels. Asmentioned above a pixel is selected for output when both a column and arow corresponding to the location of the pixel are selectedsimultaneously. The row selecting logic is similar to the one used forselecting columns but additional logic is used for resetting rows ofpixels. The control logic 140 consists of building blocks 142. Eachbuilding block contains the necessary logic for selecting one or tworows 144 at a time depending on the state of the mode input 146. The rowselecting sequence of the control logic 140 is started by a pulse inrow_ena input signal 148. If the mode signal is in the logic low state,one row is selected at a time. In this mode the signal propagatesthrough the gate 149 so that the output of each flip flop is connectedto the input of the next flip flop. At each clock cycle the signalpropagates from one flip flop to the next, selecting one column at atime. At each clock cycle 150 the next row is selected until the lastrow is reached. The row_out signal 152 is generated when both the lastrow (row_(M)) and the last column (col_(N)) are both selected.Therefore, the input to the D-flip flop producing the row_out signal 152is generated from the selection signal of the last row (row_(M)) and theselection signal for the last column (col_(N)) 156 by a logical ANDgate.

If the mode signal 148 is in the logic high state, the control logicselects two rows at a time and advances in steps of two at each clockcycle 150. In this mode the selection signal propagates through the gate157 instead of gate 149, grouping the inputs of the two flip flops ineach building block together so that they change state at the same time,i.e. two rows are selected at a time. At each clock cycle, the selectionis advanced at two row steps. Using same mode for rows and columns, 2×2pixels are grouped into one super pixels when the mode signal is in thelogical low state. Any number or combination of modes for selecting rowsand columns can be used instead or in addition to the two modes used in140.

The reset signals 158, which are used for resetting a row of pixels to apredefined value, are produced from the corresponding row selectionsignals 144 by delaying them one clock cycle using D-flip flops 159. Theoperating mode for grouping pixels does not effect the operation ofreset logic in any way. If grouping is used, several rows of pixels arereset simultaneously.

FIG. 8 shows an example of an imaging system 160 consisting of an arrayof imaging devices. The array consists of M×N imaging devices, connectedtogether to form a continuous imaging area. Each imaging device 162 inthe array of imaging devices includes the logic for programmableresolution described above. This example illustrates the connectionsbetween devices 162 using the enable and out signals for rows andcolumns, which are available in each imaging device. The imaging systemis started using the row_enable 164 and column_enable 166 signals. Oncestarted, the first row and the first column of the first imaging device,marked U_(1,1), is selected for output. The column selector advances ateach clock cycle and when the last column of the first row is read, acol_out signal 168 is produced. The col_out signal 168 is connected tothe col_ena input 170 of the next device in the array. The reading ofpixels is continued from the first row of the next device and so onuntil the last pixel in the first row of the whole imaging area is read.The col_out signal 168 of the last imaging device in each row isconnected back to the first device in the same row, as shown in FIG. 8,and the row selector is advanced. The following rows are read in thesame way until last row of the imaging device is reached and the row_outsignal 172 is given a pulse enabling the first column in the next devicein vertical direction. The row_out signal 172 of each device isconnected to the row_ena signal 174 of next device in verticaldirection. Each time the row selector is advanced to the next row, thepreviously selected row is reset. The whole area in the array of imagingdevices is read using the same method. At the moment the last pixel inthe whole area is read and a pulse is present at the col_out signal 168and row_out signal 172 of the last device, marked U_(M,N), the sequenceis started from the beginning provided that the exp_mode signal 176 isin a logic high state. The exp_mode signal 176 controls the running modeof the array. With the exp_mode signal 176 in low state, the systemoperates in single exposure mode reading the whole image area once. Newrow_enable 164 and column_enable 166 signals are required for startinganother reading sequence. On the other hand, the same system can be usedfor continuously updating live video image by applying logic high levelto the exp_mode signal 176. As long as the exp_mode signal 176 remainshigh, the reading sequence is started from the beginning once the lastpixel in the whole area is reached.

In addition to the inputs shown in the FIG. 8, clock input and the modeinputs for selecting the resolution are required, but for simplicitythey are not shown in this example. Each of the imaging devices alsoincludes the necessary circuitry for the programmable resolutiondescribed earlier. In case of using grouping of pixels for lowering theresolution, the read sequence is identical to the one described aboveexcept for the fact that several columns and rows are selectedsimultaneously and advanced in steps larger than one pixel according tothe size of the pixel group.

The imaging system in FIG. 8 can also be divided into two or moresections, each consisting of an array of imaging devices. In sucharrangement each section is connected in the same way as described abovefor the whole imaging area. Such an arrangement produces more than onesimultaneous output channels instead of just one.

An imaging device for radiation imaging system has been described. Theimaging system consists of an array of several imaging devices. Thenumber of devices in the array can be chosen according to therequirements of the target application, whether it is a small areasystem or a large area system. An imaging device as described herein hasa capability to change the resolution of the device while the system isoperating. The grouping of pixels into larger area super pixels isachieved by selecting several pixels at the same time. A pixel isselected when the row and column corresponding to the location of thepixel are selected simultaneously. With the method of operationexplained herein, several columns and rows can be selected concurrently,thus selecting a group of pixels instead of just one. The grouping ofpixels has the advantage that the same system can be used forhigh-resolution still images as well as for lower resolution videoapplications. By grouping pixels directly in the imaging device, theamount of data is significantly reduced. Also, the speed and memoryrequirements for the image acquisition and processing system are reducedconsiderably.

The imaging device described herein can easily be applied for small andlarge area applications by connecting the end of row and end of columnsignals to the corresponding row enable and column enable signals on thenext device in the array in read order.

In view of the foregoing description it will be evident to a personskilled in the art that various modifications may be made within thescope of the invention.

The scope of the present disclosure includes any novel feature orcombination of features disclosed therein either explicitly orimplicitly or any generalisation thereof irrespective of whether or notit relates to the claimed invention or mitigates any or all of theproblems addressed by the present invention. The applicant hereby givesnotice that new claims may be formulated to such features during theprosecution of this application or of any such further applicationderived therefrom. In particular, with reference to the appended claims,features from dependent claims may be combined with those of theindependent claims and features from respective independent claims maybe combined in any appropriate manner and not merely in the specificcombinations enumerated in the claims.

What we claim is:
 1. An imaging device for radiation imaging, saiddevice comprising a two-dimensional array of detector cells forgenerating a charge in response to incident radiation, an array of cellcircuits for accumulating charge generated, and control circuitrycontrolling output of signals from said cell circuits, said controlcircuitry operable to select an individual column and row of said arrayof said cell circuits, utilizing respective row and column enablesignals for said device, and is further operable to output a signalindicative of an end of row and a signal indicative of an end of column,wherein said end of row and end of column output signals of said imagingdevice are connectable to corresponding row and column enable signalinputs of a separate imaging device in first and second orthogonaldirections, respectively.
 2. An imaging device according to claim 1,wherein said array of detector cells and said array of cell circuitsform an array of pixels.
 3. An imaging system, comprising a plurality ofimaging devices according to claim 1 connected as a one ortwo-dimensional array.
 4. An imaging system according to claim 3,comprising a plurality of imaging devices connected as a two-dimensionalarray, whereby said imaging system provides selectable imagingresolutions for selected applications.
 5. An imaging system comprising aplurality of imaging devices according to claim 1, wherein said imagingdevices are disposed in a regular two-dimensional array extending insaid first and second orthogonal directions, said end of row outputsignal of a first imaging device being coupled to a row enable signalinput of a second imaging device adjacent said first imaging device insaid first direction, and said end of column output signal of said firstimaging device being coupled to a column enable signal input of a thirdimaging device adjacent said first imaging device in said seconddirection.
 6. An imaging system according to claim 5, wherein controlcircuitry is arranged to permit reading of cell circuits of one row ofpixels across multiple imaging devices from the two-dimensional array ofimaging devices, before proceeding to a subsequent row.
 7. A method ofoperating an imaging device for radiation imaging, wherein the imagingdevice includes an array of detector cells for generating a charge inresponse to incident radiation, an array of cell circuits foraccumulating charge generated, and control circuitry controlling outputof signals from said cell circuits, said method comprising: providingseparate enable signals for selecting an individual column and row ofsaid array of said cell circuits, utilizing respective row and columnenable signals for said imaging device, and providing signals indicativeof an end of row and an end of column on respective outputs of saidimaging device, for connecting said signals indicative of an end of rowand an end of column to a separate imaging device to said imagingdevice.
 8. A method according to claim 7, further comprising providingoutput signals indicative of an end of row or end of column for saidimaging device.
 9. An imaging system comprising a plurality of imagingdevices connected as a one or two dimensional array, wherein a firstimaging device for radiation imaging comprises a two-dimensional arrayof detector cells for generating a charge in response to incidentradiation, an array of cell circuits for accumulating charge generated,and control circuitry controlling output of signals from said cellcircuits, said control circuitry operable to select and individualcolumn and row of said cell circuits, utilizing respective row andcolumn enable signals for said device, and further operable to output asignal indicative of an end of row and a signal indicative of an end ofcolumn, wherein said end of row and end of column output signals of saidimaging device are connected to corresponding row and column enablesignal inputs of a separate imaging device to said first imaging device.